Flame plating by means of detonation using a detonating gun (D-Gun) has been used in industry to produce coatings of various compositions for over a quarter of a century. Basically, the detonation gun consists of a fluid cooled barrel having a small inner diameter of about one inch. Generally a mixture of oxygen and acetylene is fed into the gun along with a comminuted coating material. The oxygen-acetylene fuel gas mixture is ignited to produce a detonation wave which travels down the barrel of the gun whereupon the coating material is heated and propelled out of the gun onto an article to be coated. U.S. Pat. No. 2,714,563 discloses a method and apparatus which utilizes detonation waves for flame coating. The disclosure of this U.S. Pat. No. 2,714,563 is incorporated herein by reference as if the disclosure was recited in full text in this specification.
In general, when the fuel gas mixture in a detonation gun is ignited, detonation waves are produced whereupon the comminuted coating material is accelerated to about 2400 ft/sec and heated to a temperature about its melting point. After the coating material exits the barrel of the detonation gun a pulse of nitrogen purges the barrel. This cycle is generally repeated about four to eight times a second. Control of the detonation coating is obtained principally by varying the detonation mixture of oxygen to acetylene.
In some applications, such as producing tungsten carbide-cobalt based coatings, it was found that improved coatings could be obtained by diluting the oxygen-acetylene fuel mixture with an inert gas such as nitrogen or argon. The gaseous diluent has been found to reduce or tend to reduce the flame temperature since it does not participate in the detonation reaction. U.S. Pat. No. 2,972,550 discloses the process of diluting the oxygen-acetylene fuel mixture to enable the detonation-plating process to be used with an increased number of coating compositions and also for new and more widely useful applications based on the coating obtainable. The disclosure of this U.S. Pat. No. 2,972,550 is incorporated herein by reference as if the disclosure was recited in full text in this specification.
Generally, acetylene has been used as the combustible fuel gas because it produces both temperatures and pressures greater than those obtainable from any other saturated or unsaturated hydrocarbon gas. However, for some coating applications, the temperature of combustion of an oxygen-acetylene mixture of about 1:1 atomic ratio of oxygen to carbon yields combustion products much hotter than desired. As stated above, the general procedure for compensating for the high temperature of combustion of the oxygen-acetylene fuel gas is to dilute the fuel gas mixture with an inert gas such as nitrogen or argon. Although this dilution lowers the combustion temperature, it also results in a concomitant decrease in the peak pressure of the combustion reaction. This decrease in peak pressure results in a decrease in the velocity of the coating material propelled from the barrel onto a substrate. It has been found that with an increase of a diluting inert gas to the oxygen-acetylene fuel mixture, the peak pressure of the combustion reaction decreases faster than does the combustion temperature.
In copending U.S. patent application Ser. No. 110,841, filed Oct. 21, 1987, a novel fuel-oxidant mixture for use with an apparatus for flame plating using detonation means is disclosed. Specifically, this reference discloses that the fuel-oxidant mixture for use in detonation gun applications should comprise:
(a) an oxidant and PA1 (b) a fuel mixture of at least two combustible gases selected from the group of saturated and unsaturated hydrocarbons. PA1 .sigma..sub.t =total stress PA1 E=coating elastic modulus PA1 .epsilon..sub.f =strain-to-fracture from four point bend test PA1 .sigma..sub.r =coating residual stress, measured from blind hole test (by convention compressive stresses are negative values)
The invention also relates to an improvement in a process of flame plating with a detonation gun which comprises the step of introducing desired fuel and oxidant gases into the detonation gun to form a detonatable mixture, introducing a comminuted coating material into said detonatable mixture within the gun, and detonating the fuel-oxidant mixture to impinge the coating material onto an article to be coated and in which the improvement comprises using a detonatable fuel-oxidant mixture of an oxidant and a fuel mixture of at least two combustible gases selected from the group of saturated and unsaturated hydrocarbons. The detonation gun could consist of a mixing chamber and a barrel portion so that the detonatable fuel-oxidant mixture could be introduced into the mixing and ignition chamber while a comminuted coating material is introduced into the barrel. The ignition of the fuel oxidant mixture would then produce detonation waves which travel down the barrel of the gun whereupon the comminuted coating material is heated and propelled onto a substrate. The oxidant disclosed is one selected from the group consisting of oxygen, nitrous oxide and mixtures thereof and the like and the combustible fuel mixture is at least two gases selected from the group consisting of acetylene (C.sub.2 H.sub.2), propylene (C.sub.3 H.sub.6), methane (CH.sub.4), ethylene (C.sub.2 H.sub.4), methyl acetylene (C.sub.3 H.sub.4), propane (C.sub.3 H.sub.3), ethane (C.sub.2 H.sub. 6), butadienes (C.sub.4 H.sub.6), butylenes (C.sub.4 H.sub.8), butanes (C.sub.4 H.sub.10), cyclopropane (C.sub.3 H.sub.6), propadiene (C.sub.3 H.sub.4), cyclobutane (C.sub.4 H.sub.8) and ethylene oxide (C.sub.2 H.sub.4 O). The preferred fuel mixture recited is acetylene gas along with at least one other combustible gas such as propylene.
Plasma coating torches are another means for producing coatings of various compositions on suitable substrates. Like the detonation gun process, the plasma coating technique is a line-of-sight process in which the coating powder is heated to near or above its melting point and accelerated by a plasma gas stream against a substrate to be coated. On impact the accelerated powder forms a coating consisting of many layers of overlapping thin lenticular particles or splats. This process is also suitable for producing tungsten carbide-cobalt based coatings.
Although good tungsten carbide cobalt based coatings can be obtained from the above processes, it is not apparent upon examining the coated articles how they will react when subjected to cyclic loading. It has been found that coated articles when subject to cyclic loading can fail due to what is called fatigue. Fatigue is the progressive phenomenon of failure that occurs in materials when they are subjected to cyclic loading at stresses having a maximum value less than the tensile strength of the materials. Fatigue can generally culminate in fracture after a sufficient number of cyclic loadings. Since fatigue causes materials to fail sooner and/or at lower loads than would be expected, its net effect has been to either shorten the useful life period of materials at the same load or reduce the allowable load for the same life period.
It is an object of the present invention to provide tungsten carbide-cobalt based coatings for various substrates such that the coated articles exhibit good fatigue characteristics.
It is another object of the present invention to provide tungsten carbide-cobalt based coatings having high strain-to fracture values that result in good fatigue characteristics for articles coated with the coatings.
It is another object of the present invention to provide an improved tungsten carbide-cobalt based coating on an article in which the coating is peened by the deposition process and which coated article exhibits improved-fatigue characteristics.
It is another object of the present invention to provide an improved tungsten carbide cobalt based coating on a substrate that has been peened so that some compressive residual stresses are introduced into the surface of the substrate and which coated article exhibits improved fatigue characteristics.
The foregoing and additional objects will become more apparent from the description and disclosure hereinafter.